It has long been known to acoustically log open wellbores to determine the velocities of compressional ("P") waves, shear ("S") waves, and tube ("T") waves traveling through rock formations located in the wellbore region. Logging devices have been used for this purpose which normally comprise one or more sound sources (transmitters) and one or more receivers disposed at preselected distances from the sound source. One such borehole logging tool is described in U.S. Pat. No. 4,383,308 to R. L. Caldwell.
By timing the travel of compressional waves, shear waves and/or tube waves between the transmitter and each receiver, it is normally possible to determine the nature of surrounding rock formations. In logging loosely consolidated formations, however, it is often difficult to distinguish between compressional, shear, tube and secondary waves which may comprise portions of a wave train arriving at a given receiver. The use of remotely spaced, multiple receivers is thus intended to aid in distinguishing between arriving wave fronts and from noise in the system. Multiple receivers permit the recognition of similar wave patterns and wave fronts which are received at each successive receiver. Since travel time differences increase with increasing distance from the transmitter source, wave fronts and patterns which are closely spaced at proximate receiver locations will separate by the time of their receipt at remote receiver locations.
In the design of logging tools, various types of transmitters, such as piezoelectric or magnetostrictive transmitters, have been suggested for creating acoustic logging signals. For conventional logging operations, most such transmitters have been centrally located in the borehole and have been adapted to generate sound which is radiated in a multidirectional (360.degree.) pattern from the transmitter to adjacent wellbore surfaces. Such transmitters are well suited for creating compressional waves in surrounding rock and sand formations.
Since compressional waves travel faster than those shear, tube or secondary waves which may also be produced by a multidirectional transmitter, calculation of compressional wave velocity is accomplished by presuming that the first arriving wave front or wave pattern is that of a compressional wave. In loosely consolidated formations, subsequent arrivals of shear waves, tube waves and/or secondary waves are difficult to distinguish. In such formations, multidirectional transmitters tend to generate compressional waves of much greater amplitudes than any shear waves also produced thereby. Recognition of shear wave arrivals is thus particularly difficult.
Recently, attention has been directed to developing transmitters which are particularly suited to a single point force application of acoustic energy to the borehole wall. The theory behind point force transmitters is that they produce an asymmetrical acoustic energy radiation pattern as contrasted with the multidirectional radiation pattern. One such point force transmitter is the bender-type disclosed in U.S. Pat. No. 4,649,525 to Angona and Zemanek. Use of such a bender-type transmitter is employed in the acoustic well logging tool of U.S. Pat. No. 4,718,046 to Medlin to generate such an asymmetrical acoustic energy radiation pattern. In such logging tool the unrestricted planar surfaces of the bender-type transmitter are oriented along the longitudinal axis of the tool and are exposed to a coupling liquid within the tool. Sine wave tone bursts excite the transmitter to generate asymmetric acoustic energy waves having a dominant mode of vibration. The frequency of the tone burst may be fixed or swept as the tool traverses the subsurface formations to produce the desired compressional, shear, or tube waves.